add combo_box::State::insert

[edwloef]

I have a bunch of entries for a combo box streaming into my app from the background. I'd like these options to be in sorted order in the combo box, however combo_box::State::into_options -> slice::partition_point -> Vec::insert -> combo_box::State::new loses the current search state, which can be disorienting when a new option arrives while searching. With this method present, I can use combo_box::State::options -> slice::partition_point -> combo_box::State::insert to maintain both sorted order and search state.